Hot rolled copper or copper alloy rod used in the manufacture of wire is usually formed into coils for convenient handling. The rod accumulates a surface scale or oxide when exposed to the atmosphere, and the scale is of variable composition but usually includes a mixture of cuprous (red) and cupric (black) oxides. Before the rod with such an oxide scale is used in the manufacture of wire, the scale should be completely removed so that the wire drawn from the rod will not contain patches of oxide, and so the oxides will not form scratches and surface pits on the wire, and to increase the working life of the drawing tools used in the formation of the wire.
In order to remove surface scale or oxide from the surface of copper base products, it has become common practice to "pickle" the products by contacting their surfaces with a pickling liquid, such as a solution including sulphuric acid, nitric acid, or other acids. One of the common methods of pickling a copper product is to immerse the product in a heated bath of pickling liquid, such as a 20 percent solution of sulphuric acid for a period of time up to 30 minutes. When pickling copper or copper base rod, the rod is usually formed into a loosely coiled package with its annulus of a density of approximately 25 percent of the density of the rod and the rod is immersed in a pickling bath. The low density package allows the bath to circulate between the coils of the rod so that the acid solution contacts all surfaces of the rod and functions to dissolve the oxide. If the annulus of coils is more dense, the coils must be pulled apart in the bath to assure proper liquid circulation. The dilute sulphuric acid readily dissolves the black oxide component of the scale, but only slowly attacks the red oxide, and leaves harmful deposits of copper powder and undecomposed scale on the rod. When the coils of rod are raised from the pickling bath, a high velocity stream of liquid must be applied to the coils to beat a portion of the remaining red oxide from the surface of the rod. The coils are subsequently immersed in water to remove the acid from the surface of the rod. When the surface of the copper rod has been deoxidized in this manner, the rod can be stored and handled for extended periods of time, maybe 4 to 6 weeks, substantially without reoxidizing.
Other descriptions of pickling copper products are set forth in U.S. Pat. No. 2,291,201 and in WIRE, Coburg, Germany, Issue 90, August, 1967.
While the foregoing process of immersing copper rod in a pickling bath to remove surface oxide from the rod has been found to be successful to a limited extent, this pickling process is expensive in that it requires the use of heating equipment, large tanks of acid solution and water, handling equipment, plant space, and operators to perform the process.